Update docs for AnimationClip (#31915)

* Update AnimationClip.html

Update the documentation to add a description of the userData property for AnimationClip

* Update AnimationClip.html

Update the Chinese documentation to add a description of the userData property for AnimationClip.

docs/api/en/animation/AnimationClip.html

@@ -72,6 +72,12 @@
 			of this clip instance. It gets automatically assigned and shouldn't be
 			edited.
 		</p>
+		
+		<h3>[property:Object userData]</h3>
+		<p>
+			An object that can be used to store custom data about the animation clip.
+			It should not hold references to functions as these will not be cloned.
+		</p>
 
 		<h2>Methods</h2>
 

docs/api/zh/animation/AnimationClip.html

@@ -57,6 +57,11 @@
 
 		</p>
 
+		<h3>[property:Object userData]</h3>
+		<p>
+			一个用于存储该剪辑的自定义数据的对象，它的属性不应该持有对函数的引用，因为这些引用不会被克隆。
+		</p>
+
 
 		<h2>方法</h2>